Since the steam doesnt have a specific shape, this can be achieved only by playing around. Add a transparent layer about the food items, drag a grey color line, go to Filter-Liquify and play around to get the steam effect. Reduce the opacity as required. Blurring also gives the reality. You can also use the Shear command to give the curves for the steam. Play with your imagination !
Adobe Certified Expert, Photoshop CS